The pre-coated steel sheet is generally produced with a two-coat two-bake system in which an undercoat paint is coated on a base steel sheet as typified by a zinc-based plated steel sheet after carrying out a chemical forming treatment as a pre-treatment, heated and cured and then a top-coat paint is coated, heated and cured. Pre-coated steel sheets which have been produced in this way are delivered initially to the user in the form of a coil. Hence the user unwinds the coil and cuts off the amount which is required for the application and works this into various shapes for use.
The pre-coated steel sheet is cut and subjected to working after the paint film has been formed and so there are end surfaces and often cracks and scratches in the worked parts where the metal is exposed locally and a lowering of corrosion resistance and peeling of the paint film are liable to arise in such parts. Hence, in the past the base steel sheet has been subjected to a chemical forming treatment which includes chromate and, moreover, an undercoat paint which contains a chromium-based anti-rust pigment has been applied in order to ensure corrosion resistance and adhesion of the paint film on the base steel sheet.
However, in recent years the adverse effects on the environment due to the dissolving out of highly toxic chromium have been regarded as a problem and there is a demand for chromium-free paint compositions in which no chromium-based anti-rust pigment is used as undercoat paints.
Paints in which vanadium compounds are used as anti-rust pigments are known as chromium-free paint compositions (for example Patent Documents 1 to 4).
A chromium-free paint composition which is characterized by containing silica particles which have a specified average particle diameter, oil uptake and pore volume and magnesium salts which include magnesium vanadate has been disclosed in Patent Document 1. Furthermore, a chromium-free paint composition in which metal silicate and a metal hydrogen phosphate are used conjointly with at least one type of vanadium compound selected from among vanadium pentoxide, calcium vanadate and magnesium vanadate as anti-rust pigments has been disclosed in Patent Document 2.
Furthermore, a chromium-free paint composition in which molybdenum oxide, metal silicate, phosphoric acid-based metal salt and at least one type of vanadium compound from among vanadium pentoxide, calcium vanadate and magnesium vanadate are used conjointly as anti-rust pigments has been disclosed in Patent Document 3.
However, the chromium-free paint compositions described in Patent Documents 1 to 3 provide poor corrosion resistance when compared with paints in which chromium-based pigments are used and the corrosion resistance on the end surfaces in particular is inadequate. Furthermore, the water resistance is often poor when anti-rust pigments are used in large amounts and so this has not resulted in the substitution of chromium-based anti-rust pigments in the production of pre-coated metal sheets. Furthermore, the processes are complicated since a plurality of anti-rust pigments must be added.
Moreover, it is disclosed in Patent Document 4 that paint films with which corrosion resistance and moisture resistance are improved can be formed by setting the conductivity and pH of a 1 mass % aqueous solution of calcium vanadate in an anti-rust paint composition which contains calcium vanadate as an anti-rust pigment within specified ranges.
However, the paint compositions described in Patent Document 4 provide poor long-term corrosion resistance in corrosive environments which have a high wet rate such as salt-water spray tests when compared with paints in which chromium-based pigments are used, and the corrosion resistance in worked parts and at the end surface parts in particular is unsatisfactory. Moreover, when used for a coated steel sheet where the base material comprises 55% Al—Zn-based melt plated steel sheet, white rust and red rust formation is formed from the end surface part even in the early stages of corrosion depending on the pH of the calcium vanadate and satisfactory corrosion resistance is not obtained and so this has not resulted in the substitution of chromium-based anti-rust pigments in the production of pre-coated metal sheets.